1. Field of the Invention
The present invention relates, in general, to ladder-type bulk acoustic wave filters and, more particularly, to a ladder-type bulk acoustic wave filter with a common ground inductor, which obtains high attenuation in a high frequency stop band adjacent to a pass band without the insertion loss degradation of the pass band, so that the ladder-type bulk acoustic wave filter is useful as a transmission stage filter.
2. Description of the Related Art
Recently, with the development of mobile communication technology and the popularization of mobile communication terminals, the small-size and light weight of the parts of mobile terminals are required.
To comply with the requirements, for a filter that is one of important passive parts of the mobile terminals, a conventional bulk ceramic filter has been replaced by a more smaller surface acoustic wave filter or bulk acoustic wave filter.
Especially, there has been further popularized the bulk acoustic wave filter that has been found to have a low loss, a high selection ratio, and high power durability properties compared to the surface acoustic wave filter, and to be able to meet ultrahigh frequencies up to 10 GHz.
Such a bulk acoustic wave filter is implemented in such a way that a plurality of Film Bulk Acoustic Resonators (FBARs) are connected to each other. Bulk acoustic wave filters have been classified into a ladder-type filter in which a plurality of resonators are electrically connected and a stacked crystal filter in which a plurality of resonators are mechanically connected, and researched thereby.
In the above filters, the ladder-type filter is first commercialized. As shown in FIG. 1, the ladder-type filter is constructed so that a plurality of resonators F11 to F13 are arranged in series between an input terminal IN and an output terminal OUT, and resonators F21 and F22 are arranged between contact points of the respective resonators F11 to F13 and a ground terminal GND. In this case, the resonators F11 to F13 arranged in series with the input terminal IN and the output terminal OUT are designated as series resonators, and the resonators F21 and F22 connected to the ground GND are designated as shunt resonators.
In the above description, all of the resonators F11 to F13, F21 and F22 have different resonant frequencies designed to correspond to respective required pass bands. Generally, the series resonators are designed to have resonant frequencies higher than those of the shunt resonators by approximately 3% of a center frequency. Further, the series and shunt resonators show band pass characteristics with respect to certain frequency bands by generated poles and zeros.
FIG. 2 is an enlarged top view showing an actual bulk acoustic wave filter implemented using the circuit of FIG. 1, wherein series resonators F11, F12 and F13 are sequentially arranged and connected between input and output terminals IN and OUT, and shunt resonators F21 and F22 are arranged between the series resonators F11, F12 and F13 and the ground terminal GND.
Generally, as shown in FIG. 3, there are several factors representing the characteristics of a band pass filter, such as a center frequency fc, which is a center frequency value of a pass band, a bandwidth (BW) of the pass band, roll-off (Roll-off) representing an interval between a 3 dB attenuation point and a pole, a signal insertion loss (Loss) representing an average loss of the pass band, ripple (Ripp) representing the amplitude of a ripple component appearing in the pass band, and an attenuation ratio (Att) representing the attenuation of a signal existing outside the pass band.
In this case, as the insertion loss (Loss), the roll-off (Roll-off) and the ripple (Ripp) are smaller and the attenuation ratio (Att) is larger, the desirable characteristics of the band pass filter are obtained.
The characteristics of the conventional bulk acoustic wave filter are described on the basis of the above factors. That is, as shown in FIG. 4, the conventional bulk acoustic wave filter is constructed in such a way that shunt resonators 46 and 47 are grounded through respective bonding wires 48 and 49 when a plurality of resonators 43 to 47 are connected in a T-shape between an input terminal 41 and an output terminal 42.
In this way, if ground paths between the shunt resonators 46 and 47 are isolated from each other, attenuation characteristics can be somewhat improved in a low frequency stop band adjacent to a pass band. However, the attenuation characteristics are not significantly improved in a high frequency stop band. Therefore, the conventional bulk acoustic wave filter is not suitable for a transmission stage filter requiring high attenuation in the high frequency stop band.